1. Field of the Invention
The present invention relates to a surface finishing method and a machine for the method, in which a rotary tool is employed to finish a surface of a workpiece having a bottom and a slope continuing thereto. More specifically, the present invention relates to a surface finishing method for finishing a surface of a concave or convex portion of which profile is carved to a mold material using an end mill or the like, and a machine for implementing the method.
2. Description of Related Art
Conventionally, when a surface of, for example, a concave portion of which profile is carved to a workpiece is finished using a rotary tool such as an end mill, the surface is ordinarily carved while the end mill is revolved along a side of the concave portion and is moved gradually from an outermost side to an inner side of the concave portion. Subsequently, the end mill is moved to a bottom of the concave portion through the same movement path and the bottom is finished.
For instance, as shown in FIG. 7, when a surface of a workpiece W having a bottom 101 and a slope 103 provided successively to the bottom 101 at an obtuse angle through a slope 102, an end mill EM is first revolved along an outermost side of the slope 103 (counterclockwise in FIG. 7) to carve the surface of the workpiece W. Subsequently, the end mill EM is moved toward inner side of the slope 103 at a predetermined amount and the surface is carved again while the end mill is revolved along the slope 103. In this way, the end mill EM is gradually moved toward inside to carve the surface. The successive bottom 101 is also carved toward inner side of the bottom 101 by repeating similar movement.
However, an up-cut (cutting upward) and a down-cut (cutting downward) can be co-existed in the above conventional carving method on account of a relation between the relative movement path of the end mill EM and the workpiece W and the revolving direction (moving direction on the surface) of the end mill EM.
More specifically, the surface is down-cut (cutting downward) when the end mill EM is revolved along a side of the slope 103 to carve the surface of the workpiece in FIG. 7, since the end mill EM touches the workpiece W in a far side in the figure and a relative movement direction of a surface of the workpiece W relative to end mill EM and revolving direction (moving direction on the surface) is equal.
On the other hand, the surface is up-cut (cutting upward) when the bottom 101 is carved, since the workpiece W is carved sequentially toward inside of the concave portion and the end mill EM touches the workpiece W at a front side of the tip end of the end mill EM in the figure, thereby making the relative moving direction of the surface of the workpiece W relative to the end mill EM opposite to the revolving direction (moving direction on the surface) of the end mill EM.
When the up-cut (cutting upward) and the down-cut (cutting downward) co-exist, it is difficult to secure sufficient finish surface accuracy in all of the carved surfaces and the tool's life span can be decreased.
Generally speaking, following difference can be observed between the up-cut and the down-cut.
As shown in FIG. 8(A), in conducting the up-cut (cutting upward), the edge initially touches a surface which has been carved and gradually cuts uncarved portion, in other words, a swarf thickness in conducting the up-cut (cutting upward) starts from zero and increases gradually. In this case, when the workpiece is begun to be carved, it is impossible to cut the workpiece and the edge just rubs the surface of the workpiece since the swarf thickness, i.e. the amount by which the edge cut the workpiece, is zero. When the swarf thickness gets sufficiently large, the workpiece is substantially carved. During the time, the edge is rubbed on the surface of the workpiece with a certain amount of pressure being applied, which causes abrasion of edges and burn on the surface of the workpiece.
On the other hand, as shown in FIG. 8 (b), the edge in conducting down-cut (cutting downward) first greatly cut into an uncarved portion and gradually decreases cutting amount, in other words, a swarf thickness in conducting the down-cut (cutting downward) is the greatest at the beginning of cutting and is decreased gradually. Though the swarf thickness is zero at a final carving stage, the edge moves to separate from the workpiece and scarcely rubs the surface of the workpiece, thereby hardly causing edge abrasion and burn on the surface of the workpiece.
Therefore, the up-cut (cutting upward) is shorter in tool's life and is more likely to cause grinding burn than the down-cut (cutting downward). So, when the up-cut (cutting upward) and the down-cut (cutting downward) coexist in a workpiece as in a conventional carving method, it is difficult to obtain good finish surface accuracy in all of the carving surfaces and the tool's life is shortened.